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NaI calibrationNaI calibration and neutron observationneutron observation during the charge exchange experiment

1. Improving the NaI energy resolution (as low as reasonably achievable!)• Common noise reduction • Intercalibration• Clustering algorithm

2. Observation of the prompt signal of “high energy” neutrons (8.9 MeV)• A “matter-of-fact” evidence, in Xe and in NaI• Comparison with cross sections• First look at and requirements of a MC for neutrons in LXe.

Giovanni Signorelli, INFN Pisa MEG collaboration meeting, PSI 9 Feb 2004

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1. Common noise reduction

2. Crystal intercalibration

3. Clustering for energy summation

NaI calibration procedure

23% 11% FWHM @ 55 MeV

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Common noise reduction

Algorithm:Simplif. From E.Frlez, D. Pocanic, S.Ritt NIM A463 (2001)

1. Take the ADC of the channels which see pedestal

2. Make the average

3. Subtract it from all channels (second pedestal correction)

The pedestal ’s shrink from 56 to 23.It’s not perfect but compatible with the ALARA principle

Correlation between channels due to electronics, noise in cables, ADCs…

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Crystal intercalibration

ROUGH CALIBRATION

•Cosmic ray runs can be used to inter-calibrate crystals

•Muons triggered by crystal pairs

•Position of the Landau peak

FINE TUNING

•Problems for crystals at the center (the crystal are not uniformly spanned by cosmics?)

•Refined with monoenergetic gammas

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Energy clustering

The cluster C includes the element of the detector with the maximum energy plus all the fired elements connected to another member of the cluster by a side or a corner

E = EiiC

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Results

•The resolution is acceptable

•The peak position is well reproduced

54.8 MeV 83 MeV 129.8 MeV

5.5% 5.1% 4.9%

Reconstructed peak

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A better NaI helps

•A cleaner separation of the two NaI peaks helps in reducing the tails on the Lxe distributions

•An improved collinearity requirement shows the real performance

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Neutron observation during the experiment

•Evidence for a prompt signal from neutrons

•8.9 MeV neutron in coincidence with the 129 MeV gamma

•Neutrons from the Am/Be source (10 MeV)

•Comparison with cross sections (physics)

•Inelastic scattering

•Xe level excitation

•First look at and requirements of a MC for neutrons in LXe.

•Geant 3.21 + GCALOR

•Geant 4

•Possible use of neutrons for calibration/monitoring purposes (Angela)

•Availability – switchability

•Probe of the entire detector

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Evidence

•Runs triggered with one of the detectors only (&S1 &RF…)

•Emeasured> 110 MeV selection of the - p n events

•No timing cut (implies an energy/position cut!)

XeXe NaINaI

50% efficiency

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Neutron-induced prompt signal in Xe

For fast neutrons (110 MeV) the total and scattering cross sections are similar for all isotopes

= 1 barn = 72 cm in LXe

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Neutron cross section

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Processes

A COMPLETE MONTECARLO CALCULATION IS NEEDED FOR COMPUTING THE NEUTRON EFFECTS IN THE CALORIMETER :• efficiency for fast and thermal neutron detection• determination of the energy spectrum in the calorimeter• energy released as a function of time• energy density (x,y,z)• dependence on threshold and n-energy

ALL THE RELEVANT NEUTRON CROSS-SECTIONS CAN BE INCLUDED IN GEANT 3.21 AND ARE INCLUDED IN GEANT4

information from medical physics…….!KERMA COEFF. (Kinetic Energy Released per unit Mass) and

tr / (mass energy transfer coefficient)tabulated for neutrons

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MC for neutrons in liquid Xenon

Though the most reliable simulation today is GEANT4, some quick results were obtained with GEANT 3.21 + GCALOR

•8.9 MeV neutron simulated impinging a 10 x 10 cm2 window of the Lproto (time cut-off at 600ns) coming from the LH2 target

•GCALOR (MICAP, En < 20 MeV) takes care of n cross sections (ENDF VI B)

•N,n n,2n …

•If the residual nucleus is left in an excited state the deexcitation photon is generated (this is not done in the n,Xe n’Xe case. Bug? We generated these photons by hand)

•Some refinement still possible

•In GEANT4 the code for the neutron transportation is automatically embedded in the package and is “benchmarked” with a comparison to real data!

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Neutron Monte Carlo event sample

Incoming neutron

•8.9 MeV neutron

•10 x 10 cm2 window

•Coincidence with the 129 MeV photon

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MC spectrum

2.2 MeV capture on protons

Xe levels

•A neutron edge is present

•Low energy lines due to Xe and/or other nuclear levels

•High energy tails: n capture and isotope production

•The comparison with the data is good but not excellent

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Conclusion

• A calibration procedure for the NaI has been estabilished and coded in the (Pisa version of the) analyzer, obtaining a fairly good E resolution for this detector

• The neutron prompt signal was identified in Xe and NaI and the understanding of the process is under way. We’ll do our best to reproduce the experimental result…

• A new window is open, a new handle is present. To us the difficult task to exploit it (calibration, monitoring…)!

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…timing

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neutron

gamma

n2n

n 1 n 2

n

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Xe 129

TOT SC

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Xe 129

n2n n3n

Initial energy degradation and neutron duplication

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Xe 129 nuclear level excitations n1 n2 etc.Levels 0.039 0.236 0.318 MeV

energy degradation and kinetic energy into energy

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Xe 129 n

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Xe 132nuclear level excitations n1 n2 etc.

Levels 0.628 0.1.298 01.44 MeV

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Xe 132 n